The well-mixed greenhouse gases have lifetimes long enough to be relatively
homogeneously mixed in the troposphere. In contrast, O3 (Section
6.5) and the NMHCs (Section 6.6) are gases with
relatively short lifetimes and are therefore not homo-geneously distributed
in the troposphere.

Spectroscopic data on the gaseous species have been improved with successive
versions of the HITRAN (Rothman et al., 1992, 1998) and GEISA databases (Jacquinet-Husson
et al., 1999). Pinnock and Shine (1998) investigated the effect of the additional
hundred thousands of new lines in the 1996 edition of the HITRAN database (relative
to the 1986 and the 1992 editions) on the infrared radiative forcing due to
CO2, CH4, N2O and O3. They found
a rather small effect due to the additional lines, less than a 5% effect for
the radiative forcing of the cited gases and less than 1.5% for a doubling of
CO2. For the chlorofluorocarbons (CFCs) and their replacements, the
uncertainties in the spectroscopic data are much larger than for CO2,
CH4, N2O and O3, and differ more among the
various laboratory studies. Christidis et al. (1997) found a range of 20% between
ten different spectroscopic studies of CFC-11. Ballard et al. (2000) performed
an intercomparison of laboratory data from five groups and found the range in
the measured absorption cross-section of HCFC-22 to be about 10%.

Several previous studies of radiative forcing due to well-mixed greenhouse
gases have been performed using single, mostly global mean, vertical profiles.
Myhre and Stordal (1997) investigated the effects of spatial and temporal averaging
on the globally and annually averaged radiative forcing due to the well-mixed
greenhouse gases. The use of a single global mean vertical profile to represent
the global domain, instead of the more rigorous latitudinally varying profiles,
can lead to errors of about 5 to 10%; errors arising due to the temporal averaging
process are much less (~1%). Freckleton et al. (1998) found similar effects
and suggested three vertical profiles which could represent global atmospheric
conditions satisfactorily in radiative transfer calculations. In the above two
studies as well as in Forster et al. (1997), it is the dependence of the radiative
forcing on the tropopause height and thereby also the vertical temperature profile,
that constitutes the main reason for the need of a latitudinal resolution in
radiative forcing calculations. The radiative forcing due to halocarbons depends
on the tropopause height more than is the case for CO2 (Forster et
al., 1997; Myhre and Stordal, 1997).

Not all greenhouse gases are well mixed vertically and horizontally in the
troposphere. Freckleton et al. (1998) have investigated the effects of inhomogeneities
in the concentrations of the greenhouse gases on the radiative forcing. For
CH4 (a well-mixed greenhouse gas), the assumption that it is well-mixed
horizontally in the troposphere introduces an error much less than 1% relative
to a calculation in which a chemistry-transport model predicted distribution
of CH4 was used. For most halocarbons, and to a lesser extent for
CH4 and N2O, the mixing ratio decays with altitude in
the stratosphere. For CH4 and N2O, this implies a reduction
in the radiative forcing of up to about 3% (Freckleton et al., 1998; Myhre et
al., 1998b). For most halocarbons, this implies a reduction in the radiative
forcing up to about 10% (Christidis et al., 1997; Hansen et al., 1997a; Minschwaner
et al., 1998; Myhre et al., 1998b) while it is found to be up to 40% for a short-lived
component found in Jain et al. (2000).

Trapping of the long-wave radiation due to the presence of clouds reduces the
radiative forcing of the greenhouse gases compared to the clear-sky forcing.
However, the magnitude of the effect due to clouds varies for different greenhouse
gases. Relative to clear skies, clouds reduce the global mean radiative forcing
due to CO2 by about 15% (Pinnock et al., 1995; Myhre and Stordal,
1997), that due to CH4 and N2O is reduced by about 20%
(derived from Myhre et al., 1998b), and that due to the halocarbons is reduced
by up to 30% (Pinnock et al., 1995; Christidis et al., 1997; Myhre et al., 1998b).

The effect of stratospheric temperature adjustment also differs between the
various well-mixed greenhouse gases, owing to different gas optical depths,
spectral overlap with other gases, and the vertical profiles in the stratosphere.
The stratospheric temperature adjustment reduces the radiative forcing due to
CO2 by about 15% (Hansen et al., 1997a). CH4 and N2O
estimates are slightly modified by the stratospheric temperature adjustment,
whereas the radiative forcing due to halocarbons can increase by up to 10% depending
on the spectral overlap with O3 (IPCC, 1994).

Radiative transfer calculations are performed with different types of radiative
transfer schemes ranging from line-by-line models to band models (IPCC, 1994).
Evans and Puckrin (1999) have performed surface measurements of downward spectral
radiances which reveal the optical characteristics of individual greenhouse
gases. These measurements are compared with line-by-line calculations. The agreement
between the surface measurements and the line-by-line model is within 10% for
the most important of the greenhouse gases: CO2, CH4,
N2O, CFC-11 and CFC-12. This is not a direct test of the irradiance
change at the tropopause and thus of the radiative forcing, but the good agreement
does offer verification of fundamental radiative transfer knowledge as represented
by the line-by-line (LBL) model. This aspect concerning the LBL calculation
is reassuring as several radiative forcing determinations which employ coarser
spectral resolution models use the LBL as a benchmark tool (Freckleton et al.,
1996; Christidis et al., 1997; Minschwaner et al., 1998; Myhre et al., 1998b;
Shira et al., 2001). Satellite observations can also be useful in estimates
of radiative forcing and in the intercomparison of radiative transfer codes
(Chazette et al., 1998).